The invention relates to a device for measuring the concentration of toner in a developer mixture, particularly in an electro-photographic copying apparatus wherein the developer consists of toner and a ferromagnetic carrier material. More especially, the measuring device has a component which is part of a measuring oscillator and carries an inductance, and into the zone of inductance is passed at least a part of the developer mixture, which is to have the change in its toner concentration measured.
In known copying machines, in which the latent electrostatic images on the surface of a photo-conductive layer are developed with the aid of a developer mixture consisting of toner and a ferromagnetic carrier material, only the toner particles are consumed, whereas the ferromagnetic carrier material remains and can be re-used. It is therefore necessary to replenish the developer mixture periodically or continuously by replacing the consumed toner so that the concentration of the toner in the developer mixture is kept at a predetermined value within close tolerances, in order to achieve a desired density of the developed copies. If the concentration of toner in the developer mixture is lower than the optimum concentration, the density of the developed copy becomes inadequate, and if, on the other hand, the concentration of toner is too high, copies with an undesirable background result.
German Offenlegungsschrift No. 2,014,430 discloses a device of the type initially set forth for monitoring and controlling the concentration of toner in a developer mixture. In this device samples of the developer mixture are taken past the windings of a coil which is located in a circuit which carries an a.c. voltage. The output signals of the circuit depend on the inductance of the coil and hence on the concentration of toner in the samples of the developer mixture. There is also provided an assembly for determining the concentration of toner, which assembly is coupled to the circuit carrying the a.c. voltage and responds to the output signals. The arrangement for taking the samples of the developer mixture past the windings of the coil is a sample tube which consists of non-magnetic material and leads through the coil.
The system, described in German Offenlegungsschrift No. 2,525,952, for monitoring the concentration of toner in a developer is constructed in a similar manner and has a channel with an inlet and outlet orifice through which the developer is to be fed and a measuring coil which at least partially surrounds the channel and is connected to measuring instruments for measuring the changes of inductance in the measuring coil as a measure of the change in the concentration of toner in the developer.
German Offenlegungsschrift No. 1,936,815 relates to a device for measuring the concentration of toner in a developer mixture, consisting of a coil having a ferromagnetic core which, together with a part of the developer mixture, forms a magnetic circuit of the coil. The coil and its magnetic circuit form an inductance, the change of which influences the output frequency of an oscillating circuit so that any change in the permeability of the developer mixture results in a change in frequency. The core of the coil is in contact with the outside of the wall of the developer container which must consist of a non-magnetic material, at least in this zone, so that the magnetic field can penetrate through the wall into the developer container and the magnetic circuit of the coil is closed via that part of the developer mixture which is opposite the core.
The common feature of these known devices for measuring the concentration of toner is that the leakage inductances used reach partially into the zone of the developer mixture and that only samples of the developer mixture are covered. These are statistical measurements which do not establish with certainty whether the measured value is representative for the average of the concentration of toner in the developer mixture. A further disadvantage is that the coils used have a certain overall height which permits installation only at certain points of the copying machine and that the small internal diameter of the coils, or the tubes which lead through the coils, give rise to blockages with developer mixture, so that frequent cleaning becomes necessary.